Impact of low temperature on interactions and rheology of dense casein micelles dispersions

Caseins are present in the milk in form of casein micelles – porous, deformable and compressible colloidal particles [1, 2]. Understanding of interactions between the casein micelles and properties of concentrated casein micelles dispersions (e.g., compressibility, concentration of gel formation, gel cohesion) is important for optimization of milk filtration because appearance of the gel on filtration membrane surface reduces the filtrate flow. Bouchoux et al. [3-5] demonstrated that concentrated dispersions of casein micelles can be obtained and their properties can be characterized with the help of osmotic stress and swelling-redispersion experiments. It was found that casein micelles dispersions turn from the liquid state into the solid-like gel state at the micelles concentration of ≈ 180 g∙L–1 [3, 4] and that cohesiveness of obtained gels strongly depends on their concentration [5]. These findings allowed to introduce the maximal critical casein micelles concentration on the membrane surface, and to define milk filtration conditions that do not lead to severe membrane fouling [6].In previous experiments of Bouchoux and co-workers [3-6], casein micelles were characterized at the temperature of 20°C. However, there is an increasing interest to milk filtration at low temperature (≈8-12°C). Also, it is known that casein micelles have temperature-responsive porosity and chemical composition in diluted dispersions [1, 2]. Therefore, it is necessary to elucidate the influence of the temperature on compression, structure formation and swelling-redispersion of concentrated dispersions of casein micelles and to relate it with optimal filtration conditions.The aim of the current work was to analyse the impact of temperature on interactions and cohesion within concentrated casein micelles dispersions. The experiments were performed at 7°C and 20°C. Casein micelles dispersions with different casein micelles concentrations (from ≈120 to ≈700 g∙L–1) were obtained and casein micelles compressibility was characterized via osmotic stress experiments (from 0.05 to 13 bar). The cohesiveness of obtained gels was characterized via swelling-re-dispersion experiments: gel is introduced in milk solvent, then hydration and casein micelle re-dispersion rate are followed in function of swelling-re-dispersion duration (from 4 to 240 h). In order to distinguish the influence of the temperature on the gel structure and swelling kinetics, each gel sample was re-dispersed at two different temperatures (7°C and 20°C).It was observed that compressibility (concentration attained at given osmotic pressure) of casein micelles increases with the temperature increasing from 7°C to 20°C. This can be explained by the reduction of hydrophobic bonds at lower temperature [2]. It was found that swelling of casein micelles gels at 7°C results in lower redispersion rate as compared to the swelling at 20°C regardless of the gel formation temperature. This can be partially explained by higher thermal motion at 20°C. However, it was also observed that casein micelles gels obtained at 7°C are more cohesive than those prepared at 20°C, since for high osmotic pressures, gels are not at all re-dispersed at 7°C, while they are almost fully re-dispersed, if prepared at 20°C. This can imply that more rigorous filtration conditions should be applied during the low temperature milk filtration